The present invention relates to the design and manufacture of high pressure discharge lamps using ceramic arc tubes. Furthermore, the invention relates to a ceramic metal halide lamp which requires very little change in the manufacturing technology. The field covers any discharge lamp in a ceramic envelope, of any shape, size, power and configuration.
Metal halide lamps in ceramic arc tubes are relatively new entrants in the field of lighting. Since they can be operated at higher temperatures than lamps with quartz arc tubes, they are capable of better performance in measures such as luminous efficacy, color rendering and color stability. The recurring difficulty is obtaining a reliable seal between the arc tube ceramic and the electrical feed through.
High pressure sodium lamps (HPS) employ niobium as the feed through material since its thermal coefficient of expansion (TCE) is well matched to that of alumina. It is joined to the alumina by a ceramic sealing compound of similar thermal expansion coefficient as PCA (polycrystalline alumina) or niobium. The sealing compound is also resistant to sodium attack at elevated temperatures during lamp operation.
Without extensive modifications, this arrangement is unsuitable for ceramic metal halide (CMH) lamps since the salts are corrosive to both the niobium and sealing compound even at the lower cold spot temperatures usual for metal halide lamps. Consequently, a variety of attempts have been made and reported to overcome the sealing problem in CMH lamps.
Metals which are resistant to halides and may be used as feed throughs are molybdenum, tungsten, platinum, rhodium, rhenium, etc. These refractory metals, however, have a lower TCE than that of alumina (Table 1). Large differences in TCE result in separation between the metallic feed through and the ceramic arc tube body especially under thermal cycling during lamp operation and life. The separation causes seal leaks and even fracture leading to loss of hermeticity. Various methods of adaptation have been reported to overcome the thermal mismatch problem.
In general, the sealing methods for the feed through to the arc tube body can be divided into one or more of the following four categories: sealing compound, sintering, graded seal and new arc tube materials. In many cases, the categories overlap in practice (for example, the use of graded plug material to effect a seal by sintering).
The most common design of CMH lamp arc tubes includes a PCA tube with narrow diameter capillary tube end sections. This construction results in lower temperature in the seal area during lamp operation. The electrode feed through is in three parts, a small diameter niobium rod and tungsten electrode at either end bridged by a halide resistant middle section. The middle section may be a molybdenum rod and/or coil or cermet. A ceramic sealing compound that is more halide resistant than the one used in HPS lamps makes the seal between the PCA and niobium rod. A protective layer over the niobium rod is formed by the melted sealing compound itself. This arc tube construction makes use of the well known HPS type sealing method (alumina to niobium via sealing compound) and processes (glove box sealing) with sufficient modifications to enable a long life CMH lamp. Other CMH arc tube constructions that make use of different sealing methods such as direct sintering of PCA to feed through, use of cermets and graded seals or even the use of new arc tube materials that will enable straight sealing with molybdenum or tungsten have been reported. There have been occasional introductions of lamps that used a cermet to replace niobium. But these alternate methods have not yet been able to demonstrate an overall advantage in areas of improved lamp performance, lower cost or adaptability to existing lamp factory processes.
An object of this invention is to substitute a part most prone to halide attack in the standard ceramic metal halide arc tube construction while minimizing the thermal stress between the new feed through and PCA. Another object of this invention is to reduce the manufacturing cost of a CMH lamp.
In the ceramic metal halide lamp designs utilized most widely, niobium is used as the feed through material in order to enable a hermetic seal to the PCA. While its TCE is most favorable for plugging a PCA arc tube, it cannot withstand the very corrosive reactions with the metal halide constituents within the arc tube. Hence, extraordinary steps are taken to minimize these reactions. The seal area is located far from the arc zone in order to lower the temperature substantially. Secondly, the surface area of the niobium feed through is reduced by changing to a small diameter rod from the tubular form common in HPS lamps. Further, the exposed niobium rod within the arc tube is protected by the melted sealing compound. This construction, although successful, comes at a great price due to the necessity for a three part electrode and the difficulty of assembling the same. Additionally, lamp design freedom is restricted by the choice of metal halides that are compatible with this construction.
If niobium can be eliminated from CMH lamps, there is potential for enormous reduction in the manufacturing costs and great possibilities for new lamps as well, although thermal stress and corrosion must be considered. Different types of HID lamps were analyzed to suggest alternatives for the CMH seal design. For example, it has been found by many investigators that molybdenum in either tube or rod form was unsuitable as a feed through for PCA bodies because of the large mismatch in the TCE. However, molybdenum is used as feed through in such long lived lamps as mercury vapor and quartz metal halide lamps despite the mismatch in TCE between molybdenum and quartz or hard glass being much greater than between molybdenum and PCA (TCE of quartz xcx9c0.5xc3x9710xe2x88x926/K).
In quartz metal halide lamps, the thermal stress is kept to such a low level that it does not cause seal failure. The molybdenum at the sealing location is in the form of a thin foil whose thickness is much less than the diameter of a rod that would be required to carry the same current. In addition, the foil edges are feathered or beveled thus shaping the ends to point edges of negligible thickness.
According to the present invention, it was found that a similar approach may be adapted for CMH lamps as well. The molybdenum foil takes the place of niobium rod for sealing to PCA. The PCA capillary bore was modified so that a slit was formed at the outer end that would accept a molybdenum foil section. The width of the slit is substantially the same as the width of the molybdenum foil section. The diameter of the capillary bore is between about 0.5 and 3.0 mm. The electrode feed through assembly was made similar in appearance and construction as those in quartz metal halide lamps. Unlike with quartz, sealing between the PCA and the molybdenum foil cannot be made by melting and pressing the alumina. Instead, a sealing compound was used. Such sealing compounds comprise alumina and one or more of other oxides of silicon, dysprosium, strontium, barium, yttrium, calcium, etc. In practice, HPS lamp sealing compounds contain alumina, calcium, yttria, strontia, etc., while CMH lamp sealing compounds are usually made up of alumina, silica, dysprosia, etc. The arc tube had a good hermetic seal after the sealing process with good adherence of the sealing compound to the molybdenum foil.
A lamp was made with the arc tube of the new construction. It was operated for hundreds of hours where it was cycled on and off repeatedly. It was surprisingly found that the lamp operated without any seal failure. The present invention is based on the above discovery.